Error correction system for position measuring device

ABSTRACT

A length or angle measuring system is provided in which for the purposes of error correction the graduation of the measuring scale is scanned on both sides of the graduation axis which runs along the measuring direction. The weighting given to the two sides of the measuring graduation is varied according to the desired error correction course. Several different approaches to varying this weighting are disclosed, including the mounting of a scanning unit so as to be shiftable transversely to the graduation axis of the graduation.

BACKGROUND OF THE INVENTION

This invention relates to a length or angle measuring system of the type comprising a measuring graduation which defines a graduation axis extending along the graduation and a scanning unit positioned to scan the graduation, in which means are provided for coupling the graduation and the scanning unit to two relatively movable objects. In particular, this invention relates to an improved error correction system for such a measuring system.

A variety of error correction systems for such position measuring systems are known to the art.

For example, in U.S. Pat. No. 4,170,829 there is described a correction system for a length or angle measuring device in which a measuring graduation of the measuring device is slightly deflected at selected positions in a direction substantially perpendicular to the plane of the graduation by pressure or tension forces applied in correspondence to the desired error correction course. An array of adjusting members is mounted in the carrier body for deflecting the measuring graduation as desired. The accuracy of this nonlinear error correction system depends in part upon the number and spacing of the adjusting members per unit of measuring length.

U.S. Pat. No. 4,060,903 discloses a length measuring system in which a linear error correction is provided by means of longitudinal stretching or compression of the scale. Stretching and compression devices are provided at both ends of the measuring instrument to bring about the desired change in dimension of the scale. This correction system does not permit nonlinear error correction.

U.S. Pat. No. 4,170,828 describes a length measuring system which incorporates an error correction system which comprises a link chain. Individual members of this chain are adjustable in accordance with the desired error correction course transversely to the measuring direction. A transfer element is positioned to scan this link chain and to bring about a correcting relative movement along the measuring direction between a scanning unit (which is guided in parallel motion with respect to the graduation plane of the scale) and the scale. In this error correction system, the accuracy of the error correction is dependent upon the number of chain link members provided per unit of measuring length.

U.S. Pat. No. 4,262,423 discloses an error correction system for a length measuring instrument, in which an error correction profile is formed as an integral component of a carrier or housing for the scale. This error correction profile is scanned by the transfer element which brings about a correcting relative movement along the measuring direction between the scanning unit and the scale. The scanning unit is guided in parallel motion with respect to the graduation plane of the scale.

In the two last-mentioned correction systems, the transfer elements take the form of pivotable angle elements which are subject to mechanical wear. Such pivotable angle elements can bring about a substantial increase in the cross-sectional dimensions of the position measuring system. Such an increase in the size of the position measuring system can be detrimental to a flexible use of the measuring system in certain applications.

SUMMARY OF THE INVENTION

The present invention is directed to an improved error correction system for a position measuring device, in which deformations of the measuring scale are not required, in which the number of required mechanical elements can be reduced, and which can directly be installed in commercially standard position measuring systems without substantial structural changes to these systems.

According to this invention, a length or angle measuring system of the type described above is provided with means, included in the scanning unit, for scanning a first portion of the graduation on a first side of the graduation axis and a second portion of the graduation on a second side of the graduation axis. In addition, means are provided for varying the weighting given to the first and second portions of the graduation in accordance with the desired error correction course.

This invention provides the important advantage that it can be implemented without expensive mechanical elements, and can be used to construct a simple and economical error correction system for a position measuring device. Measuring systems incorporating the present invention can be made reliable and flexible in use because of the substantial reduction of mechanical parts which are subject to wear and because of the very small spatial size of the correction system. Furthermore, the present invention can be used to correct both linear and nonlinear errors, substantially regardless of the measuring lengths. Further advantageous features of this invention are set forth in the attached dependent claims.

The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a fragmentary top view of a length measuring instrument which incorporates a first preferred embodiment of this invention.

FIG. 1b is an elevational view in partial section taken along line 1b--1b of FIG. 1a.

FIG. 2 is an elevational view corresponding to that of FIG. 1b of a length measuring instrument which incorporates a second preferred embodiment of this invention.

FIG. 3 is an elevational view corresponding to FIG. 1b of a length measuring instrument which incorporates a third preferred embodiment of this invention.

FIG. 4 is an elevational view corresponding to FIG. 1b of a length measuring instrument which incorporates a fourth preferred embodiment of this invention.

FIG. 5 is a schematic representation of portions of a length measuring instrument which incorporates a fifth preferred embodiment of this invention.

FIG. 6 is a schematic representation of portions of a length measuring instrument which incorporates a sixth preferred embodiment of this invention.

FIG. 7 is a schematic representation of a measuring graduation and scanning elements of a length measuring instrument which incorporates a seventh preferred embodiment of this invention.

FIG. 8 is a schematic representation of a portion of a scale and scanning elements of a length measuring instrument which incorporates an eighth preferred embodiment of this invention.

FIG. 9 is a schematic representation of portions of a scale and scanning elements of a length measuring instrument which incorporates a ninth preferred embodiment of this invention.

FIG. 10 is a schematic representation of portions of a scale and scanning elements of a length measuring instrument which incorporates a tenth preferred embodiment of this invention.

FIG. 11 is a schematic representation of portions of a scale and scanning elements of a length measuring instrument which incorporates an eleventh preferred embodiment of this invention.

FIG. 12 is a schematic representation of portions of a scale and scanning elements of a length measuring instrument which incorporates a twelfth preferred embodiment of this invention.

FIG. 13 shows a portion of an evaluating unit suitable for use with the embodiments of FIGS. 7 or 8.

FIG. 14 depicts a circuit for use with the embodiments of FIGS. 11 and 12.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Turning now to the drawings, FIGS. 1a and 1b provide two views of a length measuring instrument which incorporates a first preferred embodiment of this invention. This measuring instrument includes a measuring scale 2₁ which is fastened on a bed 1₁ of a machine (not shown), for example by a suitable adhesive.

The scale 2₁ defines a measuring graduation 3₁ which is scanned by a scanning unit 4₁. The scanning unit 4₁ includes a scanning plate 5₁ which defines a scanning graduation 6₁. The scanning unit 4₁ also includes an illuminating arrangement and a plurality of photosensors (not shown). The scanning unit 4₁ is rigidly connected with a follower 7₁ which is fastened rigidly to a slide piece 8₁ of the machine.

In order to correct for measuring errors (such as division or graduation errors in the measuring graduation 3₁ or machine errors) in the measurement of the relative position between the slide piece 8₁ and the bed 1₁ of the machine, the graduation 3₁ of the scale 2₁ is made up of an array of grid lines, each oriented at an angle α₁, as are the grid lines of the scanning graduation 6₁. In this preferred embodiment, α₁ as measured with respect to the graduation axis 9₁ which runs in the measuring direction X is less than 90 degrees. The scanning unit 4₁ together with the scanning plate 5₁ are mounted to slide perpendicularly to the graduation axis 9₁ in a plane parallel to the graduation plane 10₁ of the scale 2₁ in a dovetail guide 11₁. An error correction profile 12₁ is mounted to the bed 1₁ alongside the scale 2₁ and is scanned by a roller 13₁ which is mounted to the scanning unit 4₁. A spring 14₁ is positioned between the follower 7₁ and the scanning unit 4₁ in order to bias the roller 13₁ against the error correction profile 12₁.

The error correction system of FIGS. 1a and 1b operates during the measuring process to maintain the scanning graduation 6₁ defined by the scanning plate 5₁ parallel to the measuring graduation 3₁ of the scale 2₁. However, the scanning graduation 6₁ is moved in a corrective relative movement along the measuring direction X with respect to the measuring graduation 3₁ as a result of the displacement of the scanning unit 4₁ perpendicularly to the graduation axis 9₁. Of course, the position of the scanning unit 4₁ transversely to the graduation axis 9₁ is determined by the contour of the error profile 12₁. Thus, a measuring value correction according to the desired error correction course in the measuring direction X is provided through differential weighting of the scanning of the measuring graduation 3₁ on the two sides of the graduation axis 9₁.

FIG. 2 is a view corresponding to FIG. 1b of a length measuring instrument which incorporates a second preferred embodiment of this invention. This instrument comprises a measuring scale 2₂ which is secured to a bed 1₂ of a machine (not shown), as for example by a suitable adhesive. The scale 2₂ defines a measuring graduation 3₂ which is scanned by a scanning unit 4₂. For this purpose, the scanning unit 4₂ includes a scanning plate 5₂ which defines a scanning graduation 6₂. The scanning unit 4₂ is slidably connected to a follower 7₂ which is in turn rigidly fastened to a slide piece (not shown) of the machine.

This length measuring instrument includes an error correction system. The scanning graduation 6₂ of the scanning plate 5₂ and the measuring graduation 3₂ of the scale 2₂ both run perpendicularly to the graduation axis 9₂ of the measuring graduation 3₂. The scanning plate 5₂ is mounted to move with the scanning unit 4₂, and the scanning unit 4₂ is mounted in a linear guide 11₂ of the follower 7₂ to slide at an angle α₂, which is less than 90 degrees with respect to the graduation axis 9₂. The movement of the scanning unit 4₂ is in a plane parallel to the graduation plane 10₂ of the scale 2₂. An error correction profile 12₂ is fastened alongside of the scale 2₂ on the bed 1₂ and is scanned by a roller 13₂ which is mounted to the scanning unit 4₂. A compression spring 14₂ mounted between the follower 7₂ and the scanning unit 4₂ biases the roller 13₂ against the error correction profile 12₂.

The error correction system described above acts to provide a correcting relative movement in the measuring direction X of the scanning graduation 6₂ of the scanning plate 5₂ with respect to the measuring graduation 3₂ of the scale 2₂. This correcting relative movement is brought about in consequence of the displacement of the scanning unit 4₂ at an angle α₂ to the graduation axis 9₂. In this manner, a measuring value correction according to the desired error correction course in the measuring direction X is achieved by varying the weighting with which the measuring graduation 3₂ of the scale 2₂ on both sides of the graduation axis 9₂ is scanned.

FIG. 3 shows a view corresponding to that of FIG. 1b of a length measuring instrument which incorporates a third preferred embodiment of this invention. This instrument includes a scale 2₃ which is fasten to a bed 1₃ of a machine (not shown), as for example by a suitable adhesive. This scale 2₃ defines a measuring graduation 3₃ which is scanned by a scanning unit 4₃ by means of a scanning plate 5₃ which defines a scanning graduation 6₃. In this embodiment, the scanning unit 4₃ is rigidly secured to a slide piece (not shown) of the machine.

In order to bring about the desired error correction, the graduation 6₃ of the scanning plate 5₃ and the graduation 3₃ of the scale 2₃ are oriented such that both run perpendicularly with respect to the graduation axis 9₃ of the graduation 3₃. In this embodiment, the graduation 3₃ is made up of two separate graduations 3₃ ', 3₃ " which are oriented alongside one another and are offset with respect to one another by a certain amount in the measuring direction X. This offset is unequal to (N/2)C, where N is an integer and C is the grid constant of the graduations 3₃ ', 3₃ ". In this preferred embodiment, the offset between the two graduations 3₃ ', 3_(3") is equal to (1/4)C. The scanning plate 5₃ is mounted to slide in a clamp guide 11₃ which is mounted to the scanning unit 4₃ such that the scanning plate 5₃ is guided for motion perpendicular to the graduation axis 9.sub. 3 in a plane parallel to the graduation plane 10₃ of the scale 2₃. A plurality of setting members 15₃ are arranged alongside the scale 2₃ on the bed 1₃ and are scanned by a roller 13₃ which is mounted to the scanning plate 5₃.

During the measuring process, as the scanning unit 4₃ moves along the scale 2₃, the roller 13₃ comes into contact with respective peaked surfaces of the setting members 15₃. In accordance with the position of the setting members 15₃ transversely to the graduation axis 9₃, the scanning plate 5₃ is positioned perpendicularly to the graduation axis 9₃. In this way, the scanning graduation 6₃ is moved as desired so as to scan the two graduations 3₃ ', 3₃ " on both sides of the graduation axis 9₃ with different weighting. In this way, a measuring value correction in accordance with the desired error correction course in the measuring direction X is obtained.

FIG. 4 is a view corresponding to that of FIG. 1b of a length measuring instrument which incorporates a fourth preferred embodiment of this invention. In this instrument, a scale 2₄ is fastened to a bed 1₄ of a machine (not shown) in any suitable manner. The scale 2₄ defines a measuring graduation 3₄ which is scanned by a scanning unit 4₄ by means of a scanning plate 5₄ which defines a scanning graduation 6₄. The scanning unit 4₄ is rigidly fastened to a slide piece 8₄ of the machine. In this embodiment, the graduation of the scanning plate 5₄ is oriented perpendicularly to the graduation axis 9₄ of the measuring graduation 3₄, and the graduation 3₄ of the scale 2₄ is oriented on an angle α₄ which is less than 90 degrees with respect to the graduation axis 9₄. A diaphragm 16₄ is pivotably mounted to the scanning unit 4₄ by means of a lever 17₄ in order partially to cover the graduation 3₄ of the scale 2₄ and the graduation 6₄ of the scanning plate 5₄. An error correction profile 12₄ is fastened to the bed 1₄ alongside the scale 2₄, and this profile 12₄ is scanned by a roller 13₄ which is mounted to the diaphragm 16₄. A tension spring 14₄ is provided between the lever 17₄ and the scanning unit 4₄ to bias the roller 13₄ against the profile 12₄. Thus, the error correction profile 12₄ and the roller 13₄ cooperate to position the diaphragm 16₄ transversely with respect to the graduation axis 9₄ in a accordance with the contour of the error correction profile 12₄.

During the measuring process, the diaphragm 16₄ operates to mask and partially cover a variable portion of the graduation 3₄ and the graduation 6₄ on one side of the graduation axis 9₄ in correspondence with the contour of the profile 12₄. For this reason, the graduation 3₄ of the scale 2₄ is scanned on both sides of the graduation axis 9₄ with different weighting, and in this way a measuring value correction in accordance with the desired error correction course in the measuring direction X is obtained.

FIG. 5 is a fragmentary view of a fifth preferred embodiment of this invention which operates similarly to that of FIG. 4. However, in the embodiment of FIG. 5 a scale 2₅ which defines a graduation 3₅ is scanned by a scanning plate 5₅ which defines a scanning graduation 6₅. In this embodiment, the graduation 6₅ and the graduation 3₅ are oriented perpendicularly to the graduation axis 9₅ of the graduation 3₅. Here, the graduation 3₅ is made up of two separate graduations 3₅ ', 3₅ ", each positioned on a respective side of the graduation axis 9₅ and each offset with respect to the other by a selected amount in the measuring direction X. A diaphragm 16₅ is provided and positioned to mask and cover to a variable extent portions of the graduation 3₅ ' of the scale 2₅ and the aligned portions of the graduation 6₅ of the scanning plate 5₅. During the measuring process, in consequence of the partial coverage of the graduation 3₅ ' and aligned portions of the graduation 6₅ on one side of the graduation axis 9₅, the graduation 3₅ of the scale 2₅ is scanned on both sides of the graduation axis 9₅ with different weighting, in dependence upon the position of the diaphragm 16₅. In this way, the measuring value corrected by the desired amount in the measuring direction X is provided.

FIG. 6 is a schematic representation of yet another alternate form of the embodiment of FIG. 4. As shown in FIG. 6, this embodiment includes a scale 2₆ which defines a measuring graduation 3₆, which is scanned by a scanning plate 5₆ which defines a scanning graduation 6₆. In this embodiment, the graduation 3₆ and the graduation 6₆ are both oriented perpendicularly to the graduation axis 9₆ of the graduation 3₆. As part of the error correction system of this embodiment, the graduation 6₆ is made up of two separate graduations 6₆ ', 6₆ ", each of which is oriented on a respective side of the graduation axis 9₆ and both of which are offset with respect to one another by a selected amount in the measuring direction X. In this preferred embodiment, the offset between the two graduations 6₆ ', 6₆ " is preferably equal to (1/4)C, where C is the grid constant of the two graduations 6₆ ', 6₆ ". A diaphragm 16₆ is provided to variably and partially cover portions of the graduation 3₆ of the scale 2₆ and of the graduation 6₆ ' of the scanning plate 5₆. During the measuring process, in consequence of the partial covering and masking of the graduation 3₆ and of the graduation 6₆ ' on one side of the graduation axis 9₆, the graduation 3₆ of the scale 2₆ is scanned on both sides of the graduation axis 9₆ with different weighting. In this way, a measuring value corrected according to the desired error correction course along the measuring direction X is obtained.

In alternate embodiments, a diaphragm on both sides of the graduation axis of the measuring graduation can be provided in order partially to cover the graduations of the scale and of the scanning plate to bring about the desired error correction.

If it is desired to make the error correction adjustable, the setting members 15 and the error correction profiles 12 can be made adjustable in position.

FIG. 7 shows portions of a length measuring instrument which incorporates a seventh preferred embodiment of this invention. This instrument includes a scale 2₇ which defines a graduation 3₇ which is scanned by a scanning plate 5₇ which defines a graduation 6₇. In this embodiment, the graduation 3₇ and the graduation 6₇ are both oriented perpendicularly to the graduation axis 9₇ of the graduation 3₇. The graduation 3₇ is made up of two separate graduations 3₇ ', 3₇ " which are positioned on respective sides of the graduation axis 9₇ and which are offset with respect to one another by a selected amount in the measuring direction X. The graduation 6₇ is made up of two graduation 6₇ ', 6₇ " which are not provided with any reciprocal displacement with respect to one another on the two sides of the graduation axis 9₇. Three photosensors 20₇₁ ', 20₇₂ ', 20₇₃ ' are aligned with the graduation 6₇ ', and three photosensors 20₇₁ ", 20₇₂ ", 20₇₃ " are aligned with the graduation 6₇ ". Each set of three photosensors are arranged in line along the measuring direction X. Furthermore, the photosensors 20₇₁ ', 20₇₂ ', 20₇₃ ', are not offset with respect to respective ones of the photosensors 20₇₁ ", 20₇₂ ", 20₇₃ ".

This embodiment includes an evaluating arrangement (not shown in FIG. 7) which is responsive to the output signals of the photosensors 20₇₁ ', 20₇₂ ', 20₇₃ ' to form a first sum signal. This evaluating arrangement is also responsive to the output signals of the photosensors 20₇₁ ", 20₇₂ ", 20₇₃ " to form a second sum signal. In each case, the evaluating arrangement includes means for drawing on one or more of the output signals of the respective photosensors in order to form the sum signal. Depending on the number of photosensors selected, the amplitude of the first sum signal can vary widely with respect to the amplitude of the second sum signal. In this way, the graduations 3₇ ', 3₇ " on both sides of the graduation axis 9₇ are scanned with different weighting. By causing this electronic weighting to vary depending upon the measuring position X, a measuring value correction according to the desired error course in the measuring direction X can thereby be obtained.

FIG. 8 shows portions of a length measuring system which incorporates an eighth preferred embodiment of this invention. This system includes a measuring scale 2₈ which defines a measuring graduation 3₈, which is scanned by a scanning plate 5₈ which defines a scanning graduation 6₈. In the example shown, the graduation 3₈ and the graduation 6₈ both run perpendicularly to the graduation axis 9₈ of the graduation 3₈. In this embodiment, the graduation 6₈ is made up of two separate graduations 6₈ ', 6₈ ", which are disposed on respective sides of the graduation axis 9₈ and which are offset with respect to one another by a selected amount in the measuring direction X. The graduation 3₈ is made up of two graduation 3₈ ', 3₈ " which are not reciprocally offset with respect to one another on the two sides of the graduation axis 9₈. Three photosensors 20₈₁ ', 20₈₂ ', 20₈₃ ' are aligned with graduation 6₈ ' and three additional photosensors 20₈₁ ", 20₈₂ ", 20₈₃ " are aligned with graduation 6₈ ". In each set of three photosensors, the individual photosensors are arranged one behind the other along the measuring direction X. The photosensors 20₈₁ ', 20₈₂ ', 20₈₃ ' are offset with respect to respective ones of the photosensors 20₈₁ ", 20₈₂ ", 20₈₃ " in the measuring direction in the same manner as are graduations 6₈ ', 6₈ ". Thus, the relative positions between the photosensors 20₈₁ ', 20₈₂ ', 20₈₃ ' and the graduation 6₈ ' corresponds to the relative positions between the photosensors 20₈₁ ", 20₈₂ ", 20₈₃ " and the graduation 6₈ ".

This embodiment includes an evaluating unit (not shown in FIG. 8) which is responsive to the output signals of the photosensors 20₈₁ ', 20₈₂ ', 20₈₃ ' to form a first sum signal and the output signals of the photosensors 20₈₁ ", 20₈₂ ", 20₈₃ " to form a second sum signal. According to which of the output signals of the photosensors 20₈ ' and the photosensors 20₈ " are drawn upon for the sum formation for the first and second sum signals, the graduations 3₈ ', 3₈ " are scanned on both sides of the graduation axis 9₈ with different weighting. In this way, a measuring value correction according to the error course in the measuring direction X is obtained.

FIG. 9 shows a scale 2₉ with a graduation 3₉ of a length measuring instrument which incorporates a ninth preferred embodiment of this invention. This graduation 3₉ is scanned by a scanning plate 5₉ which defines a graduation 6₉. In this embodiment, the graduation 3₉ and the graduation 6₉ are oriented perpendicularly to the graduation axis 9₉ of the graduation 3₉. Graduation 3₉ is made up of two separate graduations 3₉ ', 3₉ ", each of which is positioned on a respective side of the graduation axis 9₉, and both of which are offset with respect to one another by a certain amount in the measuring direction X. The graduation 6₉ is made up of two series of graduations, each of which includes five graduation fields 6₉₁ '-6₉₅ ', 6₉₁ "-6₉₅ ". Within each series of graduation fields, individual graduation fields are arranged in succession along the measuring direction X, and the graduation fields on reciprocal sides of the graduation axis 9₉ are not offset with respect to one another. A plurality of photosensors 20₉₁ '-20₉₅ ', 20₉₁ "-20₉₅ " are provided, each of which is allocated and aligned with a respective one of the graduation fields 6₉₁ '-6₉₅ ', 6₉₁ "-6₉₅ ", respectively. This embodiment includes an evaluating unit (not shown in FIG. 9) which forms a first sum signal from the output signals of the photosensors 20₉₁ '-20₉₅ ' and a second sum signal from the output signals of the photosensors 20₉₁ "-20₉₅ ". Depending upon which of the output signals of the photosensors 20₉ ' or of the photosensors 20₉ ' are drawn upon for the sum formation of the first and the second sum signals, the graduations 3₉ ', 3₉ " are scanned on both sides of the graduation axis 9₉ with different weighting. As before, this evaluating unit is provided with means for varying the ones of the photosensors 20₉ ', 20₉ " which are drawn upon for the formation of the respective sum signals in accordance with the stored error correction course. In this way, a measuring value correction which varies in accordance with the desired error correction course in the measuring direction X is obtained.

FIG. 10 shows a scale 2₁₀ of a length measuring instrument which incorporates a tenth preferred embodiment of this invention. The scale 2₁₀ defines a graduation 3₁₀ which is scanned by a scanning plate 5₁₀ which defines a graduation 6₁₀. In this embodiment, the graduation 3₁₀ and the graduation 6₁₀ both are oriented perpendicularly to the graduation axis 9₁₀ of the graduation 3₁₀. Here, the graduation 6₁₀ is made up of two series of graduation fields 6₁₀₁ '-6₁₀₅ ', 6₁₀₁ "-6₁₀₅ ", which fields are arranged in succession in the measuring direction X. The graduation fields 6₁₀₁ '-6₁₀₅ ' are offset with respect to respective ones of the graduation fields 6₁₀₁ "-6₁₀₅ " by a selected amount in the X direction. The graduation 3₁₀ is made up of two graduations 3₁₀ ', 3₁₀ " which are aligned with one another and are not offset with respect to one another on the two sides of the graduation axis 9₁₀. Photosensors 20₁₀₁ '-20₁₀₅ ', 20₁₀₁ "- 20₁₀₅ " are allocated and aligned with respective ones of the graduation fields 6₁₀₁ '-6₁₀₅ ', 6₁₀₁ "-6₁₀₅ ".

This embodiment includes an evaluating arrangement (not shown in FIG. 10) which forms a first sum signal from the output signals of the photosensors 20₁₀₁ '-20₁₀₅ ' and which forms a second sum signal from the output signals of the photosensors 20₁₀₁ "-20₁₀₅ ". Depending upon which of the output signals of the photosensors 20₁₀ ' or the photosensors 20₁₀ " are called upon for the sum formation for the first and second sum signals, respectively, the graduations 3₁₀ ', 3₁₀ " on both sides of the graduation axis 9₁₀ are scanned with different weighting. In this way, a measuring value correction which varies in accordance with the desired error correction in the measuring direction X is obtained.

The selection of which of the output signals of the photosensors 20 are combined to form the respective sum signal can be made for example by a computer included in the evaluating unit. For example, this computer can be programmed to store a series of correction values in computer memory which vary according to the desired error correction course along the measuring direction X. In accordance with these stored correction values, the computer can be programmed to select an appropriate number of the output signals of the photosensors 20 to be summed in order to obtain sum signals of the desired amplitude. A position measuring value can then readily be formed from the two sum signals.

FIG. 11 shows a scale 2₁₁ of a length measuring system which incorporates an eleventh preferred embodiment of this invention. The scale 2₁₁ defines a graduation 3₁₁ which is scanned by a scanning plate 5₁₁ which defines a graduation 6₁₁. In this example, the graduation 3₁₁ and the graduation 6₁₁ both are oriented perpendicularly to the graduation axis 9₁₁ of the graduation 3₁₁. In this embodiment, the graduation 3₁₁ is made up of two graduations 3₁₁ ', 3₁₁ ", each of which is disposed on a respective side of the graduation axis 9₁₁, and both of which are offset respect to one another by a selected amount in the measuring direction X. The graduation 6₁₁ is made up of two graduations 6₁₁ ', 6₁₁ " which are not offset with respect to one another on the respective sides of the graduation axis 9₁₁. Two photosensors 20₁₁ ', 20₁₁ " are aligned with the two graduations 6₁₁ ', 6₁₁ ", respectively. The photosensor 20₁₁ ' is not offset in the X direction with respect to the photosensor 20₁₁ '. In this preferred embodiment, the photosensors 20₁₁ ', 20₁₁ " can be of the type marketed by the Siemens firm as part number SINF 100.

This embodiment includes an evaluating unit (not shown in FIG. 11) which operates to vary the amplitudes of the output signals generated by the photosensors 20₁₁ ', 20₁₁ ". For example, a resistance network can be used for this purpose. In this way, the scanning of the graduations 3₁₁ ', 3₁₁ " on both sides of the graduation axis 9₁₁ is performed with different weighting. In this way, a measuring value correction is provided which varies along the measuring direction X in accordance with the desired error correction course.

FIG. 12 shows a scale 2₁₂ of a length measuring system which incorporates a twelfth preferred embodiment of this invention. The scale 2₁₂ defines a graduation 3₁₂ which is scanned by a scanning plate 5₁₂ which defines a graduation 6₁₂. In this embodiment, the graduation 3₁₂ and the graduation 6₁₂ are both oriented perpendicularly with respect to the graduation axis 9₁₂ of the graduation 3₁₂. Here, the graduation 6₁₂ is made up of two separate graduations 6₁₂ ', 6₁₂ ", each of which is disposed on respective side of the graduation axis 9₁₂, and both of which are offset with respect to one another by a selected amount in the measuring direction X. The graduation 3₁₂ is made up of two graduations 3₁₂ ', 3₁₂ " which are not offset with respect to one another on the two sides of the graduation axis 9₁₂. A photosensor 20₁₂ ', 20₁₂ " is aligned with and associated with each of the graduations 6₁₂ ', 6₁₂ ", respectively. The photosensor 20₁₂ ' is offset with respect to the photosensor 20₁₂ " in the measuring direction X by the same amount as are the graduations 6₁₂ ', 6₁₂ ".

This embodiment includes an evaluating arrangement (not shown in FIG. 12) which operates to adjust the amplitudes of the output signals of the two photosensors 20₁₂ ', 20₁₂ " as desired, for example by means of a resistance network. In this way, the two graduations 3₁₂ ', 3₁₂ " on both sides of the graduations axis 9₁₂ are scanned with different weighting. In this way, a measuring value correction according to the desired error correction course in the measuring direction X is obtained. In the embodiments of FIGS. 11 and 12, the resistance network can for example be controlled by a computer included in the evaluating unit. Preferably, this computer stores an array of correction values in computer memory which define the desired error correction as a function of the measuring position X. From these stored values, the output signals of the two photosensors can be varied in amplitude as appropriate in order to obtain the desired weighting. Position measuring values are then obtained from these weighted output signals of the photosensors.

The devices of FIGS. 1-12 include embodiments of three approaches for implementing the present invention. The embodiments of FIGS. 1-6 provide various means for physically altering the region of the measuring graduation 3 scanned by the photosensor or photosenors associated with the scanning graduation 6. In each case, the scanning signal S_(G) generated in response to light modulated by both the measuring graduation 3 and the scanning graduation 6 is a result of the summing or averaging of light which has been modulated by the measuring graduation 3 on both sides of the graduation axis 9.

The embodiments of FIGS. 7-10 do not rely on physical shifting of the position of the scanning graduations 6 or photosensors 20 with respect to the measuring graduation 3. Rather, in the embodiments of FIGS. 7-10, multiple photosensors 20 are provided on each side of the graduation axis 9. By forming the scanning signal S_(G) as a result of the summation of the photosignals generated by selected ones of the photosensors 20, these embodiments can be used to vary the weighting given to the measuring graduation 3 on the two sides of the graduation axis 9. This can perhaps best be seen in conjunction with FIG. 13. FIG. 13 shows a portion of an evaluating unit suitable for use with the embodiments of FIGS. 7 or 8. The same approach can readily be adapted for the embodiments of FIGS. 9 or 10 simply by increasing the number of elements.

As shown in FIG. 13, the six photosensors 20₇₁ '-20₇₃ '; 20₇₁ "-20₇₃ " are connected in parallel between two conductors 25, 27. Each of the photosensors 20₇₁ '-20₇₃ '; 20₇₁ "-20₇₃ " produces a respective photosignal S₁ -S₆. In this embodiment, six analog switches R₁ -R₆ are provided, each of which serves either to pass or to block a respective one of the photosignals S₁ -S₆. A scanning signal S_(G) is generated between the two conductors 25, 27. Depending upon which of the switches R₁ -R₆ is closed, this scanning signal S_(G) is formed as a summation signal of selected ones of the photosignals S₁ -S₆. When the switch R₁ is open and the switches R₂ -R₆ are closed as shown in FIG. 13, the scanning signal S_(G) equals the sum defined below:

    S.sub.G 32 S.sub.2 +S.sub.3 +S.sub.4 +S.sub.5 +S.sub.6

In effect, the photosignals S₂ and S₃ have been summed to generate a first sum signal S₁₋₃ indicative of the light amplitude modulated by the measuring graduation 3₇ ', and the photosignals S₄, S₅, and S₆ have been summed to generate a second sum signal S₄₋₆ indicative of the amplitude of light modulated by the measuring graduation 3₇ ". These two sum signals S₁₋₃, S₄₋₆ are then in turn summed to produce the scanning signal S_(G). The positions of the switches R₁ -R₆ can be controlled, for example by a computer included in the evaluating unit, on the basis of correction values stored in the computer memory in accordance with the desired error correction course. The amplitude of the total sum or scanning signal S_(G) is proportional to the sum of the photosensor areas used for the evaluation. The individual photosensor areas can be equal in size or different in size as desired.

The significance of forming the scanning signal S_(G) in the manner described above in conjunction with FIG. 13 can best be understood by recognizing that the first sum signal S₁₋₃, formed of the sum of selected ones of the photosignals S₁, S₂, and S₃, is a sinusoidal signal which takes the form S₁₋₃ =Asin(x). Similarly, the second sum signal S₄₋₆, formed of the sum of selected ones of the three photosignals S₄, S₅, S₆, is also a sinusoidal signal which takes the form S₄₋₆ =Bsin (X+β). The overall sum signal S_(G) takes the form S_(G) =S₁₋₃ +S₄₋₆ =E sin (X+γ). In the formulas of this paragraph the following symbol definitions have been used:

A=amplitude of signal S₁₋₃ ;

B=amplitude of signal S₄₋₆ ;

E=amplitude of the overall sum signal S_(G) ;

β=phase displacement or offset between the graduations 3₇ ' and 3₇ ";

γ=phase displacement of the overall sum signal S_(G) ; ##EQU1##

It can be seen from the foregoing formulas that the overall sum or scanning signal S_(G) is a sinusoidal signal having a phase displacement γ (which is the desired error correction) which is a function of the amplitudes of the two intermediate sum signals S₁₋₃ and S₄₋₆. By simply selecting which of the photosignals S₁, S₂, S₃ is included in the sum signal S₁₋₃ and similarly which of the photosignals S₄, S₅, S₆ is included in the sum signal S₄₋₆, the amplitudes A and B of the sum signals S₁₋₃, S₄₋₆ can be selected in order to vary the correction phase displacement γ as desired.

From the foregoing relationships, it should be apparent that it is preferable that the phase displacement β not be equal to 180 degrees. This is because when β is equal to 180 degrees, the desired error correction γ is equal to zero. In the presently preferred embodiment, β is equal to 90 degrees. When β is set equal to 90 degrees, γ is equal to 0 degrees for B=0 and γ is equal to 45 degrees for the case where B=A. In this way, by varying B in the range between B=0 and B=A, the correction γ can be made to vary between an error correction of 0 and an error correction of 45 degrees (i.e. 1/8 of the grid constant of the measuring grid 3₇ ', 3₇ ").

The embodiments of FIGS. 11 and 12 represent a third approach to the present invention, which does not rely on the switching in and out of individual photosensors. Rather, in the embodiments of FIGS. 11 and 12, two photosensors are used, one on either side of the graduation axis 9. However, as described above, the photosignals generated by these photosensors 20 are adjusted in amplitude in order to accomplish a variable error correction in a manner analogous to that described above.

FIG. 14 depicts a circuit suitable for use with the embodiments of FIGS. 11 and 12. As shown in FIG. 14, the photosignal S₁ generated by one of the photosensors 20₁₁ ', 20₁₂ ' is summed with the photosignal S₂ generated by the other of the two photosensors 20₁₁ ", 20₁₂ ", respectively. The photosignal S₁ is attenuated by a fixed resistance and the photosignal S₂ is attenuated by a variable resistance W. The summation of the two photosignals S₁, S₂ is the scanning signal S_(G) which is applied to an evaluating unit.

Using the terminology of the discussion above in conjunction with FIG. 13, S₁ takes the form S₁ =Asin(X) and S₂ takes the form S₂ =Bsin(X+β), where β is the phase displacement between the graduations 3₁₁ ', 3₁₁ " or 6₁₂ ', 6₁₂ ". By varying the resistance W in FIG. 14, the amplitude B of signal S₂ can be altered as desired. In this way, in a manner analogous to that described above in conjunction with FIG. 13, the scanning signal S_(G) takes the form S_(G) =S₁ +S₂ =E sin(X+γ), where γ is the phase displacement of the scanning signal S_(G).

Once the scanning signal S_(G) has been formed (as for example as shown in FIGS. 13 and 14), the scanning signal S_(G) is then evaluated in a conventional evaluating unit of the prior art. For example, the evaluating unit designated as EXE 700 manufactured by the firm of Dr. Johannes Heidenhain, Traunreut, West Germany, can be used to evaluate the scanning signal S_(G) in order to determine the relative position between the scanning unit and the scale.

It should be understood that the graduations 3₇ -3₁₂ and the graduations 6₇ -6₁₂ can be oriented to run obliquely rather than perpendicularly with respect to the graduation axis 9₇ -9₁₂. In embodiments adapted for use with more Moire scanning the graduations 3₇ -3₁₂ can enclose an angle with respect to the graduations 6₇ -6₁₂. Furthermore, the number of photosensors 20₇ -20₁₀ can be made arbitrarily large. The larger the number of photosensors is, the finer the corrections which can be made.

Ordinarily in incremental measuring instruments, four output signals offset from one another by a phase angle of 90 degrees are generated by correspondingly positioned graduation fields on the scanning plate and associated photosensors. These four output signals are used for direction discrimination of the measuring movement and for the elimination of the direct voltage constituents of the output signals of the photosensors. Any of the preferred embodiments described above can be adapted to generate four such output signals by applying the principles illustrated above to each of the four separate scanning fields or each of the four separate sets of photosensors.

Of course, it should be understood that a wide range of changes and modifications to the preferred embodiments described above will be apparent to those skilled in the art. For example, the present invention is not restricted to use with photoelectric measuring systems, but is also readily adapted for use with optical, magnetic, inductive, and capacitive measuring systems. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, which are intended to define the scope of this invention. 

I claim:
 1. In a length or angle measuring system comprising a measuring graduation which defines a graduation axis extending along the graduation in a measuring direction, and a scanning unit positioned to scan the graduation, wherein means are provided for coupling the graduation and the scanning unit to two relatively movable objects, the improvement comprising:means, included in the scanning unit, for scanning a first portion of the graduation on a first side of the graduation axis and a second portion of the graduation on a second side of the graduation axis; and means for varying the weighting given to the first and second portions of the graduation in accordance with a desired error correction pattern.
 2. The invention of claim 1 wherein the varying means comprises means for shifting the scanning unit along a path angled with respect to the graduation axis.
 3. The invention of claim 1 wherein the varying means comprises a diaphragm positioned to define a scanning field of the scanning unit, and means for shifting the diaphragm along a path angled with respect to the graduation axis.
 4. The invention of claim 1 wherein the scanning means comprises means for generating first and second scanning signals in response to the first and second portions of the graduation, and wherein the varying means operates electronically to vary the weights given to the first and second scanning signals.
 5. The invention of claim 1 wherein the varying means comprises means for shifting at least a portion of the scanning unit substantially transversely to the graduation axis.
 6. The invention of claim 3 wherein the scanning means comprises a scanning graduation and wherein the diaphragm is positioned to partially cover at least one of the measuring graduation and the scanning graduation on at least one side of the graduation axis.
 7. The invention of claim 4 wherein the means for generating first and second scanning signals comprises: at least two photosensors positioned to scan the graduation on the first side of the graduation axis to generate at least two first scanning signals; at least two additional photosensors positioned to scan the graduation on the second side of the graduation axis to generate at least two second scanning signals; and wherein the varying means comprises means responsive to a subset of the first scanning signals for generating a first sum signal and means responsive to a subset of the second scanning signals for generating a second sum signal, wherein the subsets of the first and second scanning signals are selected in accordance with the desired error correction pattern.
 8. The invention of claim 4 wherein each of the scanning signals is generated by a respective first and second photosensor, and wherein the varying means operates to vary the amplitudes of the first and second scanning signals.
 9. The invention of claim 5 wherein the measuring graduation comprises a plurality of grid lines, each oriented in a graduation plane at an angle α₁ with respect to the graduation axis; wherein α₁ is an angle less than 90°; wherein the scanning unit comprises a scanning element; and wherein the shifting means moves the scanning element of the scanning unit in a plane parallel to the graduation plane, and in a direction transverse to the graduation axis.
 10. The invention of claim 1 wherein the measuring graduation comprises a plurality of grid lines, each oriented in a graduation plane and perpendicular to the graduation axis; wherein the scanning unit comprises a scanning element; and wherein the varying means comprises means for shifting at least the scanning element in a plane parallel to the graduation plane at an angle α₂ with respect to the graduation axis, wherein α₂ is an angle less than 90°.
 11. The invention of claim 1 wherein the first portion of the graduation comprises an array of first grid lines; wherein the second portion of the graduation comprises an array of second grid lines; wherein the first and second grid lines are oriented in a graduation plane perpendicular to the graduation axis; wherein the first grid lines are offset with respect to the second grid lines by a selected amount in the measuring direction; wherein the scanning unit comprises a scanning element; and wherein the varying means comprises means for shifting at least the scanning element in a plane parallel to the graduation plane and perpendicular to the graduation axis.
 12. The invention of claim 6 wherein the measuring graduation comprises a plurality of grid lines, each oriented at an angle α₄ less than 90° with respect to the graduation axis.
 13. The invention of claim 6 wherein the first portion of the measuring graduation comprises an array of first grid lines; wherein the second portion of the measuring graduation comprises an array of second grid lines; wherein the first and second grid lines are oriented perpendicular to the graduation axis; and wherein the first grid lines are offset with respect to the second grid lines by a selected amount in the measuring direction.
 14. The invention of claim 6 wherein the measuring graduation comprises an array of grid lines oriented perpendicular to graduation axis; wherein the scanning graduation defines at least an array of first scanning lines on a first side of the graduation axis and at least an array of second scanning lines on a second side of the graduation axis; and wherein the first scanning lines are offset with respect to the second scanning lines by a selected amount in the measuring direction.
 15. The invention of claim 7 wherein the means for generating first and second scanning signals further comprises a scanning element which defines a single scanning graduation aligned both with the at least two photosensors and the at least two additional photosensors.
 16. The invention of claim 7 wherein the means for generating first and second scanning signals further comprises a scanning element which defines a first scanning graduation aligned with the at least two photosensors and a second scanning graduation aligned with the at least two additional photosensors.
 17. The invention of claim 8 wherein the means for generating first and second scanning signals further comprises a scanning element which defines a single scanning graduation aligned with the first and second photosensors.
 18. The invention of claim 7 wherein the means for generating first and second scanning signals further comprises a scanning element which defines at least one scanning graduation; wherein one of the measuring graduation and the scanning graduation defines an array of first grid lines and an array of second grid lines positioned to one side of the first grid lines; and wherein the first grid lines are offset with respect to the second grid lines by a selected amount in the measuring direction.
 19. The invention of claim 8 wherein the means for generating first and second scanning signals further comprises a scanning element which defines at least one scanning graduation; wherein one of the measuring graduation and the scanning graduation defines an array of first grid lines and an array of second grid lines positioned to one side of the first grid lines; and wherein the first grid lines are offset with respect to the second grid lines by a selected amount in the measuring direction.
 20. The invention of claim 5 wherein the invention further comprises:an error profile positioned alongside the measuring graduation; and a transfer element positioned to scan the error profile and coupled to the at least a portion of the scanning unit to position the at least a portion of the scanning unit in response to the contour of the error profile.
 21. The invention of claim 6 wherein the invention further comprises:an error profile positioned alongside the measuring graduation; and a transfer element positioned to scan the error profile and coupled to the diaphragm to position the diaphragm in response to the contour of the error profile.
 22. The invention of claim 5 wherein the invention further comprises:a plurality of setting elements positioned alongside the scale; and a transfer element positioned to scan the setting elements and coupled to the at least a portion of the scanning unit to position the at least a portion of the scanning unit in response to the setting elements.
 23. The invention of claim 6 wherein the invention further comprises:a plurality of setting elements positioned alongside the scale; and a transfer element positioned to scan the setting elements and coupled to the diaphragm to position the diaphragm in response to the setting elements.
 24. The invention of claim 21 wherein the transfer element comprises a roller. 